Viscoelastic polyurethane foams, also known as slow-recovery foams, show slow recovery in a cyclic compressive process, and thus they have high hysteretic properties, usually as well as low ball rebound rates. These properties mainly relate to the intrinsic structures of foamed polymers. One method is to control the glass transition temperature of the foam around room temperature by choosing the components. Another method is to limit the speed of air flowing in and out of the foam by controlling the foam structure.
One common approach to controlling the glass transition temperature of a viscoelastic foam is to mixed use the polyether having a low hydroxyl value and the polyether having a high hydroxyl value. For example, Chinese patent applications CN1606580A, CN1572186A and CN1229803A are all based on this theory. Another approach to providing viscoelastic performance is to add monohydric alcohols. For example, Chinese patent application CN1831025A relates to using 10 to 25 parts of a monohydric alcohol having a molecular weight of 200 to 500; U.S. Pat. No. 6,391,935B1 relates to using 15 to 70 parts of a monohydric alcohol having a hydroxyl value of less than 56 mgKOH/g; and WO2010/009205A1 relates to using 1 to 20 parts of a monohydric alcohol or a mixture of multiple monohydric alcohols.
At present, the viscoelastic polyurethane foam usually has a ball rebound rate of less than 15% (as described in Chinese patent applications CN101412798A, CN101160366A, CN1922230A, etc.). Moreover, the size of the cells of viscoelastic polyurethane foams having a density of 50-70 kg/m3 is relatively large and coarse, resulting in a good air permeability but poor sound absorption performance.
Chinese patent application CN1910650A discloses a sound-absorbing system containing a viscoelastic foam, but it does not mention the preparation method of the viscoelastic polyurethane foam and its sound absorption performance.
Chinese patent application CN101903434A relates to preparing sound-absorbing material with a maximum sound absorption coefficient of 0.70 or more by using polyols derived from plants, but it does not involve any contents relating to viscoelastic foams. Chinese patent application CN101410430A describes a method for producing polyurethane foam with a density of free rise foam of 5-40 kg/m3, wherein the maximum sound absorption effect of polyurethane foam is achieved at a frequency between 1000 to 2000 Hz. Chinese patent application CN101238163A provides a method for preparing soft polyurethane foam plastics with good vibration characteristics and sound absorption performance, wherein the rebound rate is greater than or equal to 55%, the sound absorption coefficient is about 0.20 at 500 Hz, and the sound absorption coefficient is larger than 0.45 at 1000 Hz and the sound absorption coefficient is larger than 0.90 at 2000 Hz. Chinese patent application CN1849351A provides a soft polyurethane foam and its preparation method; and particularly, the foam has an improved sound absorption performance at a low frequency range, with a sound absorption coefficient of greater than or equal to 0.3 at 500 Hz and a sound absorption coefficient of greater than or equal to 0.55 at 2000 Hz. However, none of the above-mentioned patent documents mentions the viscoelastic polyurethane foam, especially the sound absorption performance of viscoelastic polyurethane foam having a ball rebound rate of 15-30%. The sound absorption performance of viscoelastic polyurethane foam having a ball rebound rate of 15-30% is not involved in the above patent documents, let alone the viscoelastic foam material with excellent sound absorption performance and the preparation method thereof.